Semiconductive light emitters based on gallium nitride and processes for making same have been known for a relatively long time; however, no instrument has been developed hitherto which would possess a stable efficiency of a controlled light emission within a broad range of spectrum (from UV to IR inclusive).
Known in the art is a semiconductive light emitter based on gallium nitride (cf. H. P. Maruska, D. A. Stevenson, Solid State Electronics, vol. 17, No. 11, 1974, pp. 1171-1179), comprising a substrate from a single-crystalline material transparent within the visible range of spectrum with applied thereonto layer of gallium nitride of the n-type of conductivity and an overlaying layer of gallium nitride alloyed with acceptor dopes. The instrument has two metallic electrodes. The metallic electrode on which a negative-polarity voltage is applied is formed on the layer of gallium nitride alloyed with acceptor dopes, while the metallic electrode onto which a positive-polarity voltage is applied is formed on the layer of gallium nitride of the n-type conductivity (in the end-face portion of the layer).
The process for the manufacture of this instrument comprises epitaxial growing, on a substrate of a single-crystalline material transparent within the visible range of spectrum, a layer of gallium nitride of the n-type conductivity, epitaxial growing, on this layer, a layer of gallium nitride alloyed with acceptor dopes; formation, on the endface portion of the first layer of a metallic electrode onto which a positive-polarity voltage is applied and, on the second layer, of a metallic electrode onto which a negativepolarity voltage is applied.
In the above-described light-emitting device the grown layers of gallium nitride comprise aggregated microscopic domains forming an integrated single-crystalline facet structure. The surface of the grown layers comprises alternating projections and recesses.
The structural imperfections and morphological characteristics of the surface of the layer of gallium nitride alloyed by acceptor dopes (alternation of projections and recesses) result in a non-uniform distribution of acceptor dopes in the layer, as well as in a non-uniform distribution of the potential over the layer surface. The acceptor dopes are thus accumulated in projections and recesses of the layers in voids between the grains.
In operation of the device (upon application of an external voltage) the above-mentioned factors cause, in the layer of gallium nitride alloyed by acceptor dopes, the formation of current leakage channels (i.e. low-ohmic regions) or irrevocable breakdown of the layers and, hence, in disappearance of the light emission.
To a certain extent the above-mentioned disadvantages have been overcome in a semiconductive light emitter based on gallium nitride (cf. French Pat. No. 2,363,900 published May 5, 1978). This device comprises a substrate from a single crystalline material transparent in the visible range of the spectrum. A layer of gallium nitride of the n-type of conductivity is applied onto the substrate. A layer of gallium nitride alloyed with acceptor dopes and consisting of two sub-layers is deposited onto the former layer. The first sub-layer is only slightly alloyed with acceptor dopes and deposited directly on the layer of gallium nitride of the n-type of conductivity, while the second active sub-layer is strongly alloyed with acceptor dopes and deposited onto the first sub-layer.
The device also has two metallic electrodes. A metallic electrode to which a negative-polarity voltage is applied is formed on an active sub-layer of the layer of gallium nitride alloyed with acceptor dopes. A metallic electrode to which a positive-polarity voltage is applied is formed on the layer of gallium nitride of the n-type of conductivity.
The process for the manufacture of this device comprises epitaxial growing, on a substrate of a single-crystalline material transparent in the visible range of the spectrum, of a layer of gallium nitride of the n-type conductivity; epitaxial growing, on this layer, of a layer of gallium nitride alloyed with acceptor dopes and consisting of two- sub-layers. First grown in the sub-layer weakly alloyed with acceptor dopes and then--the active sub-layer strongly alloyed with acceptor dopes. By way of a precision adjustment of the supply rate of the reagents, a high accuracy of maintaining the growing temperature and a small initial rate of growth of the layer of gallium nitride of the n-type conductivity a smooth surface of the active sub-layer of the layer of gallium nitride alloyed with acceptor dopes is obtained which makes it possible to ensure a more uniform distribution of acceptor impurities within the layer.
Thereafter metallic electrodes are formed. A metallic electrode onto which a negative-polarity voltage is applied is formed on the active sub-layer of gallium nitride layer alloyed with acceptor dopes. A metallic electrode onto which a positive-polarity voltage is applied is formed on the layer of gallium nitride of the n-type conductivity. The second metallic electrode is formed either on the end face of the layer of gallium nitride of the n-type conductivity, or a meso-structure is formed (i.e. the alloyed layer of gallium nitride is etched till the layer of gallium nitride of the n-type conductivity gets uncovered), whereafter on the surface of the uncovered layer a metallic electrode is formed.
As it has been already mentioned hereinabove, a relatively uniform distribution of acceptor dopes over the layer of gallium nitride improves, to a certain extent, quality of the manufactured device.
However, due to the crucial character of the process parameters in the manufacture of the device, the required high reproducibility of the results is not assured. This is due to the fact that it is very difficult to deposit a thickness-uniform thin (about 1,000 .ANG.) active sub-layer, strongly alloyed with acceptor dopes, of the layer of gallium nitride simultaneously with ensuring a uniform distribution of these dopes over the entire sub-layer. In this connection, in the active sub-layer there are low-ohmic regions which causes current leakage through the active sub-layer when the device is in operation, different values of voltage drop in various regions of the active sub-layer and color non-uniformity in light emission from diverse regions of the active sub-layer.
All this does not enable a stable efficiency of the controlled light emission in operation of the device within a broad range of spectrum (from UV to red inclusive).
Furthermore, the complexity of application of the active sub-layer and formation of metallic electrodes extends considerably the process cycle for the manufacture of the device and adds up to its manufacture costs.